Javascript: Duplicate each string value in an array

Question

Tried googling for a while but couldn't find my answer. I am trying to take a 3-character string and convert it to a 6-character string in which each of the 3 characters is duplicated.

For example, 3ff would become 33ffff.

First, I'm turning the string 3ff into the array ["3", "f", "f"].

After I converted to an array, I tried to map through the array and *2 each value, hoping to get the new array ["33", "ff", "ff"].

However, I am getting the new array [6, NaN, NaN].

I understand that 3 is getting multiplied by 2 for 6, and that JS doesn't know how to multiply strings so we get NaN.

I thought that in Python, if you try to *2 a string value, it will just duplicate the value, like eggs will turn into eggseggs. Is this not the case in JS?

Here is my code:

const duplicate = function(v) {
  newArray = v.split("");
  doubledArray = newArray.map(i => i*2);
  return doubledArray
};

console.log(duplicate("3ff")) // Output: [6, NaN, NaN]

Is there a way to duplicate string values of an array in JavaScript, and not just number values?

No it's not possible to multiply. But you can just do i => i + i instead — Alto
– Alto, Commented Mar 28, 2021 at 18:37
@Alto JavaScript, where '1'+'1' == '11'. Python, like Ruby, has string-specific overrides for *. JavaScript does not. — tadman
– tadman, Commented Mar 28, 2021 at 18:40

Jake White · Accepted Answer · 2021-03-28 18:46:13Z

3

You probably want to use .repeat() function like:

const duplicate = (v) => v.split("").map(i => i.repeat(2));
console.log(duplicate("3ff"));

Old notation:

const duplicate = function(v) {
    newArray = v.split("");
    doubledArray = newArray.map(i => i.repeat(2));
    return doubledArray
};
console.log(duplicate("3ff"));

edited Mar 28, 2021 at 18:46

answered Mar 28, 2021 at 18:41

Jake White

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1 Comment

Katie Reynolds Over a year ago

I hadn't heard of the repeat function before. Thank you! This worked perfectly.

georg · Accepted Answer · 2021-03-28 19:52:43Z

2

No need to use arrays for this:

console.log('abc'.replace(/./g, '$&$&'))

In regexp language, a dot means "any symbol" and $& = "the found symbol"

answered Mar 28, 2021 at 19:52

georg

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1 Comment

Katie Reynolds Over a year ago

Oh cool! I didn't even think that there could be a way to do this without using arrays. This is a good tip I'll keep in mind for the future.

masoudmanson · Accepted Answer · 2021-03-28 18:42:05Z

1

This can help you

const duplicate = function(v) {
  return [...v].map( i => i.toString() + i).join('');
};

console.log(duplicate("3ff"))

answered Mar 28, 2021 at 18:42

masoudmanson

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1 Comment

Katie Reynolds Over a year ago

Ahh, I see what you did there. Thanks for this suggestion! So many creative ways of doing the same thing :)

lolll · Accepted Answer · 2021-03-28 18:42:51Z

1

Try this

const duplicate = function(v) {
    newArray = v.split("");
    doubledArray = newArray.map(i => i + i);
    return doubledArray.join("")
  };

console.log(duplicate("3ff"));

answered Mar 28, 2021 at 18:42

lolll

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Comments

Best Partner · Accepted Answer · 2021-03-28 18:43:17Z

1

Please follow like this.

doubledArray = newArray.map(i => i+''+i);

Because I may be a number, so you should add string.

answered Mar 28, 2021 at 18:43

Best Partner

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4 Comments

Alto Over a year ago

string + string is always another string

Best Partner Over a year ago

But if the i is numeric value, it will be calculated.

Alto Over a year ago

Ah I see. I was looking at "3ff"

charlietfl Over a year ago

i is not a number. split('') creates array of strings

DecPK · Accepted Answer · 2021-03-28 18:51:15Z

1

You can use string concatination using string template literal.

const duplicate = function (v) {
  return [...v].map((c) => `${c}${c}`);
};

console.log(duplicate("3ff"));

answered Mar 28, 2021 at 18:51

DecPK

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1 Comment

Katie Reynolds Over a year ago

Makes sense! I would never have thought of this. So useful to see everyone's different thought processes of going through the same problem.

Katie Reynolds · Accepted Answer · 2021-03-28 20:50:52Z

0

Answered! There were many ways to do this, but here were the two simplest for me to understand:

1.) Use repeat, like this:

const duplicate = function(v) {
    newArray = v.split("");
    doubledArray = newArray.map(i => i.repeat(2));
    return doubledArray.join("")
};

console.log(duplicate("3ff")); // Output: 33ffff

2.) Use i + i, like this:

const duplicate = function(v) {
    newArray = v.split("");
    doubledArray = newArray.map(i => i + i);
    return doubledArray.join("")
  };

console.log(duplicate("3ff")); // Output: 33ffff

Thanks all for the great suggestions!

answered Mar 28, 2021 at 20:50

Katie Reynolds

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